[Bug target/79671] [7 Regression] mapnik miscompilation on armv7hl since r235622

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671

--- Comment #41 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #38)
> Simplified testcase for discussion (is not "miscompiled"):
> 
> struct S {
>   union { int i; } u;
> };
> 
> int main()
> {
>   S s;
>   new (&s.u.i) float (2.0);
>   S q = s;
>   if (*reinterpret_cast<float *>(&q.u.i) != 2.0)
>     abort ();
> }
> 
> so you say q = s is valid because the object representation of the union
> is copied.  I say after storing 2.0 to s.u.i the access 's' in q = s is
> invalid as you are accessing the stored value (a float) via a glvalue of
> inappropriate type.

I say what's wrong with this testcase is that storing a float in an int field
is undefined.